1. Field of the Invention
This invention relates to an air electrode for a solid electrolyte fuel cell and more particularly to a solid electrolyte fuel cell which uses the air electrode.
2. Related Art Statement
Recently, fuel cells have been recognized as power generating devices. The fuel cell is a device capable of directly converting chemical energy of fuel to electric energy. As the fuel cell is free from limitation of Carnot's cycle, the cell is a very promising technique owing to its inherently high energy conversion efficiency, wide latitude of fuels to be used (naphtha, natural gas, methanol, coal reformed gas, heavy oil and the like), less public nuisance, and high electric power generating efficiency without being affected by the scale of installations.
Particularly, as the solid electrolyte fuel cell (referred to as "SOFC" hereinafter) operates at high temperatures such as 1,000.degree. C., activity of electrodes is very high. Moreover, the SOFC has low polarization and relatively high output voltage without requiring any catalyst of an expensive noble metal such as platinum so that energy conversion efficiency is much higher than that of other fuel cells. Furthermore, the SOFC is stable and has long service life because all the constituent materials of the SOFC are solid.
In the SOFC, as the constituent components are solid, fuel cells of various structures have been proposed. Moreover, conductive perovskite type oxides of La(Sr)MnO.sub.3 group, La(Sr)CoO.sub.3 group and the like are expected as promising materials for the air electrode. Particularly, materials of La(Sr)MnO.sub.3 group tend to match zirconia solid electrolyte films both in reactivity and thermal expansion coefficient.
However, the materials of the air electrodes described above are higher in electric resistance than materials of the fuel electrodes, which is the reason why output of the electric cells is comparatively low. Moreover, electric current flows along thin air electrode films or air electrode substrates and in parallel therewith, and thus the electric resistance and voltage losses become great as a whole.
In order to avoid this, it is conceivable to make the air electrode as thick as possible to reduce the electric resistance. With a thick air electrode, however, gas diffusion resistance in the air electrode becomes very high, with consequent considerable decrease of oxide agent supply amount to three-phase interfaces where the air electrode, the solid electrolyte film and the oxide agent contact one another. As a result, the output of the unit cell lowers contrary to the expectation.
In order to reduce the gas diffusion resistance in the air electrode to improve the permeability to the oxide agent, it is conceivable to make large the open porosity and pore diameters of the air electrode. However, such a countermeasure will reduce the number and areas of the three-phase interfaces where the air electrode, the solid electrolyte film and the oxide agent contact one another. Moreover, the larger the open porosity, the higher the resistance of the electrode itself so that the output of the single cell lowers likewise.